High performance and safer electrolytes for lithium-ion electrochemical devices

ABSTRACT

Fire resistant stable electrolytes for use in lithium-ion based electrochemical devices, which include lithium salts in high molar concentration with various solvents, such as ethylene carbonate, and with a low % of butylene carbonate or propylene carbonate or gamma butyrolactone, and which devices may include a cathode with a lithium compound additive, and a graphitic anode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to fire resistant stable electrolytes forlithium-ion devices which have a low percent of high boiling pointsolvents in combination with ethylene carbonate and a high molar saltconcentration. These electrolytes have similar performancecharacteristics as existing highly flammable electrolytes, and have awider operating temperature span.

2. Description of the Prior Art

Prior art electrolytes that have high ionic conductivity(=high ratecapability) are very flammable due to the inclusion of low boiling pointflammable solvents such as dimethyl carbonate (DMC), diethyl carbonate(DEC), ethylmethyl carbonate (EMC), and dimethoxyethane (DME), or theyhave a low ionic conductivity due to the inclusion of high boiling pointnon-flammable glycol ethers, or they have a high percent (40% or more)of propylene carbonate (PC) or gamma-butyrolactone (GBL), which makesthem non-competitive in performance in lithium-ion batteries withgraphitic anodes, where the PC or GBL reacts with the anode, and causesa very high irreversible capacity loss, and a sharp decline in capacityduring cycling. Examples of such prior art high boiling pointelectrolytes are described in the following patents and publications:

The article entitled “Polyacrylonitrile Electrolyte Based Li-ionBatteries” published in Electrochenical Acta (1998), 43 (16-17),2399-2412 describes fire resistant polyacrylonitrile (pan) basedelectrolytes such as IM LiPF₆/EC/PC 50%, however this electrolyte doesnot have equal conductivity with flammable electrolytes and is incombination with a polymer.

The article entitled “PeoLike Polymer Electrolytes With HighRoom-Temperature Conductivity” published in the Journal of theElectrochemical society (1997), 144(6), Li36-Li38 describes fireresistant electrolytes such as IM LiPF₆/PEGDME, however, thiselectrolyte is less conductive than flammable electrolytes.

The U.S. Pat. No. 5,252,3413 of Alamgir et al. entitled “Solid PolymerElectrolyte Lithium Batteries” describes a fire resistant electrolyte IMLiCLCO₄/EC/PC 50% however it has less conductivity than flammableelectrolytes and is in combination with a polymer.

The article entitled “Laminated Thin Li-ion Batteries Using a LiquidElectrolyte” published in the Journal of the Electrochemical Society,149(1) A9-A12 (2002) describes a fire resistant electrolyte 1.5MLiBF₄/EC/GBL 75% or 1:3 ratio, which has a shorter cycle life and ahigher capacity decline than flammable electrolytes.

The U.S. Pat. No. 6,280,881 of Wendsjo et al. entitled “LithiumSecondary Battery” describes a fire resistant electrolyte IM LiBF₄/EC/PCin a 1:1 or 50/50 ratio. This electrolyte does not have the narrow ratiorange of EC, PC, or GBL or BC in combination with a polymer. Thiselectrolyte is immobilized by a polymer.

Most of the prior art fire resistant electrolytes described above areone molar in lithium salt concentration, and they are in combinationwith a polymer. It should be noted, that although the individual saltsor high boiling point liquids are known, what is not known is theoptimum narrow ratio of solvents in combination with the molar contentof the salts in the mixtures necessary to achieve an equivalentperformance to liquid flammable (lower boiling point) electrolytes inlithium-ion, or lithium-ion-polymer batteries with graphitic anodes.

The combinations described produce fire resistant electrolytes (due totheir high flash points and boiling points) that are useful inlithium-ion electrochemical devices, and provide many positiveadvantages over equivalent flammable electrolytes.

SUMMARY OF THE INVENTION

It has now been found that safe fire resistant electrolytes forlithium-ion batteries are available, which electrolytes have highconductivity, minimal reactivity and good cycle life, and which can bemade by using a lower percent of the high boiling solventgamma-butyrolactone (GBL) in combination with ethylene carbonate (EC),and a high molar concentration (M) of the salt, such as 2M of lithiumtetrafluoroborate (LiBF₄) salt, or a low percent of high boiling pointsolvents, such as propylene carbonate (PC) or butylene carbonate (BC) incombination with ethylene carbonate (EC), and a high molarconcentration, such as 1.5M to 2M of lithium tetrafluoroborate salt, ortheir mixtures.

The principal object of the invention is to provide electrolytes forlithium-ion electrochemical devices that have good ionic conductivityand are fire resistant.

A further object of the invention is to provide electrolytes of thecharacter aforesaid, which provide improved cycling stability for theelectrochemical devices in which they are incorporated.

A further object of the invention is to provide electrolytes of thecharacter aforesaid which are useful in a variety of electrochemicaldevices, such as automotive and military batteries, and capacitors.

A further object of the invention is to provide electrolytes of thecharacter aforesaid which are particularly suitable for mass production.

Other objects and advantageous features of the invention will beapparent from the description and claims.

DESCRIPTION OF THE DRAWINGS

The nature and characteristic features of the invention will be morereadily understood from the following description taken in connectionwith the accompanying drawings forming part hereof in which:

FIG. 1 is a graph of voltage versus time of a prior art flammableelectrolyte in a lithium-ion electrochemical device having thecomposition 1M LiPF₆/EC/DMC/EMC(1:1:1);

FIG. 1A is a graph illustrating capacity versus cycles of the devicewith the electrolyte therein of FIG. 1;

FIG. 2 is a graph of voltage versus time of an identical electrochemicaldevice as in FIG. 1, incorporating an electrolyte of the invention ofthe composition 2M LiBF₄/EC/GBL 20%;

FIG. 2A is a graph illustrating capacity versus cycles of the devicewith the electrolyte therein of FIG. 2;

FIG. 3 is a graph of voltage versus tine of prior art fire resistantelectrolyte in an identical electrochemical device as in FIG. 1, havingtherein the composition 1.5M LiBF4/EC/GBL 75%;

FIG. 3A is a graph illustrating capacity versus cycles of the devicewith the electrolyte therein of FIG. 3;

FIG. 4 is a graph of voltage versus time of an identical electrochemicaldevice as in FIG. 1, incorporating an electrolyte of the invention ofthe composition 1.5M LiBF₄/EC/PC 20%;

FIG. 4A is a graph illustrating capacity versus cycles of the devicewith the electrolyte therein of FIG. 4;

FIG. 5 is a graph of voltage versus tine of a prior art fire resistantelectrolyte in an identical electrochemical device as in FIG. 1, havingthe composition 1M LiBF₄/EC/PC 50%;

FIG. 6 is a chart of the conductivity of the electrolytes of FIGS. 2 and2A, for a temperature span of −25° C. to 70° C., and

FIG. 7 is a chart of the conductivity of the electrolytes of FIGS. 4 and4A, for a temperature span of −25° C. to 70° C.

It should, of course, be understood that the description and drawingsherein are merely illustrative and that various modifications andchanges can be made in the compositions disclosed without departing fromthe spirit of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

When referring to the preferred embodiments, certain terminology will beutilized for the sake of clarity. Use of such terminology is intended toencompass not only the described embodiments, but also technicalequivalents which operate and function in substantially the same way tobring about the same result.

An electrochemical device of the lithium-ion variety, such as alithium-ion cell (not shown) typically includes an anode, and a currentcollector in contact with the anode, a cathode and current collector incontact with the cathode, a separator and an electrolyte in contact withthe anode and cathode, with the whole assembly contained in a moistureproof enclosure with exiting sealed terminals.

Referring to FIGS. 1 and 1A, the results obtained with a prior artelectrolyte in a lithium-ion electrochemical device are illustrated,where the composition of the electrolyte is 1M LiPF₆/EC/DMC/EMC(1:1:1),and while it provides good performance the composition is highlyflammable.

Referring to FIGS. 2 and 2A results are illustrated which were obtainedwith an electrolyte prepared in accordance with the invention, in alithium-ion electrochemical device, with mesocarbon microbeads (MCMB) orboron coated graphite fiber anodes.

The electrolyte of the invention contained 2M LiBF₄ salt in an EC/GBL20% mixture (or 4:1 ratio) and provided similar performance as the priorart electrolyte of FIGS. 1, 1A in rate capability, and cycle life, aswell as capacity decline at C/2 rate.

Referring to FIGS. 3 and 3A the results obtained with a prior art fireresistant electrolyte electrochemical device are illustrated.

The composition of FIGS. 3, 3A was 1.5M LiBF₄/EC/GBL 75% in alithium-ion cell, with mesocarbon microbeads (MCMB), or boron coatedgraphite fiber anodes. The results obtained when compared to FIGS. 2 and2A show that the composition of the invention (FIGS. 2 & 2A)outperformed the prior art electrolyte (FIGS. 3, 3A) in less capacitydecline at C/2 rate.

Referring to FIGS. 4, 4A the results obtained with another electrolytecomposition prepared in accordance with the invention in a lithium-ionelectrochemical device are illustrated.

The composition of FIGS. 4, 4A was 1.5 M LiBF₄ in EC/PC 20%, (or 4:1ratio) in a lithium-ion electrochemical device with MCMB anode, and theresults compared to the prior art electrolyte of FIG. 5, illustratedthat it outperformed the prior art electrolyte composition of 1MLiBF₄/EC/PC 50% in cycle life, and in C/2 rate capability. (Flat tops ofthe peaks in the graph indicate high resistance=low conductivity). Bothlithium-ion cells are identical and have MCMB anodes.

The described fire resistant electrolytes have very close performance(rate and cycle life) to the existing described flammable electrolytesof FIGS. 1 and 1A, and are useable in lithium-ion andlithium-ion-polymer rechargeable batteries, and pseudocapacitors withgraphitic, or other carbon anodes.

It has also been discovered that the high boiling point solvent ECcarries most of the ionic conductivity load, and is stable with thecarbon anode, while the low percentage of high boiling point GBL or PCor BC keeps the EC in a liquid state, especially at low temperatures.The relatively high viscosity and thus lower conductivity of theirmixtures, as well as the lower conductivity of the LiBF₄ salt can beovercome by the higher molar content of the salt, which creates more ofthe necessary ionic bridges. Hence the ability to use 1.5M to 2M. Priorart 1M LiBF₄ is not good enough in these viscous electrolytes. The LiBF₄salt also has a low molecular weight as opposed to LiPF₆, or other knownsalts which makes 1.5 to 3M or more loading in the electrolyte possible,and which also helps to maintain the liquid state of the EC, even at lowtemperatures. We also have found that EC and LiBF₄ form a eutecticsolution in this range. The LiBF₄ salt is temperature resistant and lesssensitive to moisture, which are additional benefits of the electrolytesof the invention.

The above high boiling point liquids with LiBF₄ salt make theelectrolytes fire resistant under normal atmospheric (air) conditions,and temperatures created by ignition with a match, or electrical spark,for example, which makes them safer in military and automotiveapplications. The useful range of LiBF₄ is 1.5M to 3M. The useful rangeof GBL, or PC, or BC is 10% to 30% by weight percent, and preferably 15%to 25% and more preferably approximately 20%. Operating temperature ofthe above electrolytes is from (−) 20° C. to (+) 150° C., andconductivities from 0.9 mS to 10.4 mS (at 70° C.) as shown in FIGS. 6 &7. Similar mixtures as described above, but with 1M to 2.0 M LiPF₆ saltor other salts are useful, but less satisfactory. Also a combination ofLiBF₄ and LiPF₆ or other salts in the range of 0.5M to 1.5M each, areuseful. The above electrolytes are particularly useful with zeroirreversible capacity loss cathodes, as described in our prior Pat.Appl. #PCT/US02/36878, which is incorporated herein by reference.

It will thus be seen that electrolyte compositions have been providedwith which the objects of the invention are achieved.

1. A fire resistant stable electrolyte composition for lithium-ion basedelectrochemical devices which comprises: LiBF₄ salt in the range of 1.5to 3.0 molar concentration in the mixture of ethylene carbonate in therange of 70 to 90% by weight percentage, and gamma-butyrolactone in therange of 10 to 30% by weight percentage.
 2. A fire resistant stableelectrolyte composition for lithium-ion based electrochemical deviceswhich comprises: LiBF₄ salt in the range of 1.5 to 3.0 molarconcentration in the mixture of ethylene carbonate in the range of 70 to90% by weight percentage, and propylene carbonate in the range of 10 to30% by weight percentage.
 3. A fire resistant stable electrolytecomposition for lithium-ion based electrochemical devices whichcomprises: LiBF₄ salt in the range of 1.5 to 3 molar concentration inthe mixture of ethylene carbonate in the range of 70 to 90% by weightpercentage, and butylene carbonate in the range of 10 to 30% by weightpercentage.
 4. A fire resistant stable electrolyte composition forlithium-ion based electrochemical devices which comprises: two molarLiBF₄ salt concentration in the mixture of ethylene carbonate of 80% byweight percentage, and gamma-butyrolactone of 20% by weight percentage.5. A fire resistant stable electrolyte composition for lithium-ion basedelectrochemical devices which comprises: 1.5 molar LiBF₄ saltconcentration in the mixture of ethylene carbonate of 80% by weightpercentage, and propylene carbonate of 20% by weight percentage.
 6. Afire resistant stable electrolyte composition for lithium-ion batteriesand other lithium based electrochemical devices which comprises: 1.5molar LiBF₄ salt concentration in the mixture of ethylene carbonate of80% by weight percentage, and butylene carbonate of 20% by weightpercentage.
 7. A fire resistant stable electrolyte composition forlithium-ion based electrochemical devices which comprises a mixture ofelectrolytes as described in claims 1, 2 and
 3. 8. A fire resistantstable electrolyte composition for lithium-ion based electrochemicaldevices which comprises a mixture of electrolytes as described in claims4, 5 and
 6. 9. A fire resistant stable electrolyte composition asdescribed in claims 1 to 8 inclusive for lithium-ion basedelectrochemical devices in which said LiBF₄ salt is replaced by; atleast one other lithium salt in the range of 1.0 to 2.0 molarconcentration.
 10. A fire resistant stable electrolyte composition forlithium-ion based electrochemical devices which comprises: LiBF₄ salt inthe range of 1.5 to 3.0 molar concentration in approximately 100%ethylene carbonate.
 11. A fire resistant stable electrolyte compositionas described in claims 1-8 inclusive to which said LiBF₄ salt has atleast one other lithium salt added thereto in the range of 0.5M to 1.5M.12. A fire resistant stable electrolyte as described in claims 1-11inclusive, in combination with lithium-ion based based electrochemicaldevices, which have a cathode with a lithium compound additive.